Secret signaling system



arch 8, i949. C, E ATKINS ET Al. 2,463,505

SECRET SIGNLING SYSTEM Filed July 27, 1945 ZShets-Sheet 1 s, Y Q Iglu CA RL EDWARD 4T/UNS @d Y ALEXANDER l/. M/TCHELL March 8, l949,

Filed July 27, 1943 v A-E OUTPUT c. E. ATKlNs ETAL SECRET SIGNALING` SYSTEM 2 Sheets-Sheet. 2

INVENTOR CAR/ EDWARD Ark/Ns @'d ALEXANDER M M/rcf/ELL B'Y, l. A

ATTORNEYS Patented Mar. 8, 17949 NITED STTES SECRET SIGNALING SYSTEM Delaware Application July 27, 1943, Serial No. 496,272

(Cl. Z50-6) 19 Claims. l

Our present invention relates to station-tostation secret communications systems of the type wherein a transmitter and receiver at each station are suitably linked together and operate to form an end-to-end oscillator of which part of the interstage linkage comprises the intervening space between the stations. A system of this general type employing frequency modulation with signaling energy at the two stations utilized for mutual simultaneous modulation is disclosed and claimed in an application of Carl E. Atkins, Serial No. 468,345. By our present invention we have improved the system of said application by substantially reducing the equipment at each station and correspondingly simplifying the operation. The simplification effected by the present invention does not detract from the secrecy of communication nor impair the exchange of communication between the stations. As described in that application, the system included a unit for supplying the necessary signaling energy in the form of a frequency modulated sine wave, the exact character of which was controlled by a telegraph key or by speech energy. Such unit included a self-excited oscillator the frequency of which was shifted by means of a reactance tube which in turn was controlled by any suitable audio frequency energy. A special delay and phase shift lter was also provided in the System of that application to introduce into the system a delayed replica of the frequency deviations originating in the above-mentioned unit. In accordance with the present invention we are able to omit such special filter and practically all of the equipment comprising the above-described signaling unit. We have also been able to eliminate and simplify other parts of the equipment, as will be apparent as the description proceeds.

For an understanding of the improved and simplified system of the present invention, reference may be had to the accompanying drawings.

Fig. 1 is a diagrammatic circuit drawing of apparatus embodying the invention and corresponding to equipment at either of two communicating stations; and

Fig. 2 is a diagrammatic circuit drawing illustrating an alternative and preferred arrangement for suppressing, in the detector, the locally introduced frequency deviations.

As shown in Fig. l each station includes a receiving antenna 2 which feeds to a suitable frequency converter 4, an intermediate tank circuit 6 receiving energy from the converter 4, and a limiter 8 connected between the tank circuit 5 and a second frequency converter Il) wherein the frequency is increased to a value suitable for radiation by an antenna I2. A reactance tube l connected across the tank 6, is excited by audio frequency signalling energy introduced at I6 from any suitable source. A controllable portion of the audio frequency voltages appearing across a resistor i8 is applied to a second reactance tube 2E) which in turn controls the frequency of oscillation of an oscillator 22 feeding one pair of input terminals of a mixer 24, the other pair of input terminals of which are connected across the output circuit of limiter 8. Mixer 24 is any suitable circuit that delivers energy of a frequency equal to the sum of the frequencies of the energies delivered thereto, as, for example, a balanced modulator circuit. A detector 26 is connected in the output circuit of mixer 24. Oscillator 22 has an operating frequency equal to that to which the tank circuit 6 is tuned. Detector 26 may be either of the heterodyne or of the slope variety.

The apparatus at each station can be identical except for such dierences in the converters ll and l0 as are necessary to provide for transmission and reception at diiferent frequencies. For convenience, in the following description of the operation of the system the subscript a will be used to identify apparatus at one station, say station A, and the subscript b will be used to identify equipment at the other station, say station B.

Under idling conditions, that is, when audio frequency energy is not available at Ita and |612, the energy received by mixer 24 from limiter 8 will be of the same frequency as that received from oscillator 22, and hence there will be no response in detector 26. If now audio frequency energy is introduced at Ia, tube lila operates to correspondingly vary the tuning of tank circuit Sa and accordingly frequency modulate the transmission from antenna l2a. By proper adjustment of the tap 28a on resistor I 8c the reactance tube 20a can be made to receive just that portion of the audio frequency energy necessary to shift the frequency of oscillator 22a by amounts equal and opposite to the frequency changes produced in the system by the action of reactance tube I4a on tank circuit 6a. With such adjustment there will be no change in the sum frequencies passed by mixer 24a and hence locally introduced frequency deviations will not aiect the local detector. Frequency deviations appearing in tank circuit 6a as a result of the action of tube I4b on tank 6b at station B will, however, be detected at station A, as they will not be balanced out by mixer 24a. In order to insure secrecy, it will be understood that audio frequency energy will be continually available at the points I6a and Ib when communication is established between the stations. Such audio frequency energy may be speech energy or telegraphic signals with random noise energy interspersed during intervals between transmission of intelligence.

Thus with the above described systernthe signaling energy at the two stations is utilized for mutual simultaneous modulation by means of which the exchanged intelligence is completely masked. In the system, any response of the detector to locally introduced frequency deviations is prevented by the described arrangement in which the audio frequency energy initiating such deviations aifects the system frequency through reactance tube I4 and at the same time affects the frequency of oscillator 22 through reactance tube 20 to insure cancellation of such deviations in the output of mixer 24.

Fig. 2 represents a further simplification of the system in that the summation oscillator 22 and mixer 24 of Fig. l are omitted from the equipment of each station, suppression of the locally introduced frequency deviations being effected in the detecting circuit. In Fig. 2, as in Fig. 1, reactance tube I4 is shunted across the resonant circuit 6 to vary the frequency of the system in response to the bias impressed on the reactance tube by audio-frequency signal energy introduced at I6. In the particular arrangement illustrated in Fig. 2 the audio frequency biasing voltages to be applied to tubes I4 and 2D are adjustable respectively by a tap 2I on a resistor '29 and by a tap 3l on a resistor 33; resistors 29 and 33 being connected in parallel between the source of audio frequency signal energy and the ground. l In Fig. 2 the output from limiter 8 is fed to an amplifier stage 28 in the plate circuit of which is a frequency detector network of the type of the well-known Seeley circuit. The detecting circuit, as shown, includes a circuit 30 tuned to the idling frequency of circuit Ii and inductively coupled to the coil 32 of a second circuit 34 tuned to the same frequency. Coil 32 at its mid point is conductively connected to the circuit 30 through a blocking condenser 35. Diodes 36 and 38 are connected to the circuit 34 and operate in connection with series connected load resistors 40 and 42. A by-pass condenser 44 of a relatively small capacity grounds the circuit for radio frequency only. Audio frequency energy appearing across the series-connected load resistors 40 and 42 as a result of frequency excursions `applied to the circuit are passed on to any suitable audio frequency apparatus connected at 46. With the above detecting circuit we have found that the locally introduced frequency deviations may be suppressed by connecting the reactance tube directly to the circuit 34 through a condenser 4B provided that the reactance tubes I4 and 20 are excited in suitable phase and have substantially identical characteristic curves. Also the modulation should be as free as possible from, distortion as where substantial distortion occurs it is difficult to have both the fundamental and the harmonics resulting from distortion in phase opposition. If the audio frequency signal energy introduced at I6 includes frequencies commensurate with the time of signal transit, phase shifting means to retard such higher frequency audio components should be inserted between the audio frequency source and reactance tube 20 as will be apparent to those skilled in the art.

In the embodiment of the invention illustrated in Fig. 2, as well as that illustrated in Fig. 1, when the detector is of the slope variety, straight symmetrical frequency modulation is employed and hence during communication between the stations, straightforward simultaneous modulation will result and an eavesdropper will hear both messages at once. If such masking of one message by the other is not suiiicient to insure secrecy, additional camouflage couldl be provided, if desired, by applying some simple, but different, scrambling system to the audio frequency energy at cach station prior to the modulation. Where the system is used for one-Way communication of intelligence, very effective masking of the conveyed intelligence is achieved by reinjecting the modulation from one station, say station A, after a few cycles delay into the system as modulation returning from station B. Thus if ordinary speech is transmitted from station A to station B this will go around the system as ordinary-Mr intelligible frequency modulation. However, this demodulated speech could be recorded and reintroduced substantially immediately at station B, in which case the transmitted speech would be so confused that deciphering would become very diilicult if not impossible. Such reintroduction of the speech energy from station A at station B' would leave station Bs demodulation ability unimpaired and hence would not interfere with the transmission of intelligence from A to B.

The invention has now been described in connection with twospecic embodiments thereof. Obviously various changes in the particular elements described might be made Without departing from the spirit of the invention. For example, the limiter 8, while useful for eliminating amplitude variations, is not essential and could be omitted, or conversely, if desired, additional limiters could be included in the system. Furthermore, although the frequency of the system is changed by shifting at each station the bias on a reactance tube shunting a tuned circuit in the oscillatory chain, any number of reactive circuits could be so shunted and if found necessary, suitable automatic frequency control could be used in connection with the reactance tubes to insure proper alinement with the center frequency. Since all the circuits in the oscillatory chain affect the operating frequency of the system, it is desirable to utilize non-reactive couplings, such as are employed in television amplifiers or properly designed band pass filters in order to reduce to avminimum the number of interstage couplings affecting the frequency. To prevent 'frequency drift of oscillators forming part of the frequency converters 4 and I 0, means preferably such `as disclosed in the said copending application should be provided for stabilizing such oscillators. As such "frequency stabilizing means form no part of our present invention, it has not been thought necessary to describe them herein.

Unless the frequencies of transmission and reception are widely different, known shielding means should be provided vto prevent reception by antenna 2 of energy transmitted by antenna I2.

We claim:

l. In a system for secret communication between two stations the combination at each station comprising receiving and transmitting means, a pair of frequency converters, one of said converters being connected to the receiving means and the other of such converters being connected to said transmitting means, a frequency determining circuit connecting said converters, a reactance tube connected to said circuit for varying the tuning thereof, means for energizing said tube with audio frequency signal energy, an. oscillator having an operating frequency equal to the frequency of the energy passed by said circuit under idling conditions, a reactance tube for varying the frequency of said oscillator, means for energizing said last mentioned reactance tube with a controllable portion of the audio frequency signal energy and a detector responsive to variations in the sum of the frequencies of the energy passed by said circuit and the energy delivered by said oscillator whereby the detector at one station may be made responsive only to frequency deviations introduced at the other station.

2. In a station-to-station secret signalling system of the type wherein equipment at the two stations together with the intervening space form an end-to-end oscillator, the combination which compriss a frequency determining circuit at each station in the oscillatory chain, means at each station for frequency modulating said circuit in response to audio frequency signal energy,4 a detecting means at each station connected to said circuit, and means for rendering said detector unresponsive to changes in frequency of the energy passed by said circuit produced by the modulating means at the same station.

3. The combination according to claim 2 wherein said last mentioned lmeans includes a mixer connected between said circuit and said detector and a device for delivering to said mixer energy, the frequency deviations of which are responsive to the audio frequency signal energy and are equal and opposite to the frequency deviations of the energy passed by said circuit due to the means at the same station for modulating the circuit whereby the sum frequencies delivered to said detector by said mixer are unaffected by the audio frequency signal energy at the same station.

4. The combination according to claim 2 wherein said detecting means includes two circuits inductively and conductively coupled together and both tuned to the frequency under idling conditions of said frequency determining circuit, one of said circuits being connected with said frequency determining circuit and the other of said circuits having a pair of diodes and series connected load resistors associated therewith for transforming frequency deviations to audio frequency voltages and wherein said last mentioned means includes a reactance tube energized by the audio frequency signal energy and connected to said last mentioned tuned circuit to counteract such of the frequency deviations delivered to the rst mentioned tuned circuit as are caused by the audio frequency signal Ienergy at that station,

5. A station-to-station secret signalling system comprising in combination equipment at one station including a receiving device, a frequency converter, a tuned circuit, a second frequency converter and a transmitting device all connected in a chain in the order` given, like equipment at the other station, the equipment at the two stations, together with the intervening space forming an end-to-end oscillator, frequency modulating means at each station including a reactance tube energized by audio frequency signal energy and shunting the tuned circuit, detecting means at each station connected to said circuit and means for rendering said detecting means unresponsive to frequency modulation resulting from the energization of the reactance tube at the same station.

6. The system according to claim 5 wherein said last mentioned means include an oscillator, a reactance tube controlling the frequency thereof and energized by the audio frequency signal energy and a mixer receiving energy from said oscillator and from said circuit, said detecting means being connected to receive energy from said mixer.

7. The system according to claim 5 wherein said detecting means includes a pair of circuits, each tuned to the frequency passed by said circuit under idling conditions and inductively and conductively coupled together, an amplifier stage connecting one of said last mentioned circuits to said first mentioned tuned circuit, a pair of diodes and series connected resistors connected with the other of said tuned circuits cooperating to transform frequency deviations into detectable audio frequency and wherein the last mentioned means of said claim include a reactance tube energized by locally introduced audio frequency signal energy and connected to said last mentioned one of said two tuned circuits.

8. The method of secret signalling between two stations each equipped with apparatus including a detector and a pair of reactance tubes and with other apparatus including frequency determining circuits, such other apparatus forming with the intervening space therebetween an end-to-end oscillator, which comprises utilizing audio frequency signal energy at each station to simultaneously energize the pair of reactance tubes, utilizing one of the reactance tubes at each station to vary the tuning of a frequency determining circuit in the oscillatory chain at that station and utilizing the other reactance tube to render the detector at that station unresponsive to frequency modulation of the system resulting from the energization of the first reactance tube by the audio frequency signal energy.

9. The method of secret signalling between two stations each equipped with a detector, a reactance tube and with apparatus, including a frequency determining circuit shunted by the reactance tube, forming with the intervening space therebetween an end-to-end oscillator, which comprises utilizing audio frequency signal energy at each station to simultaneously frequency modulate the system by varying the tuning of the frequency determining circuit at each station in the oscillatory chain through the reactance tube shunting the circuit and energized by the audio frequency signal energy, and detecting at one station the frequency modulation introduced into the system at the other station by suppressing, in energy delivered from the frequency determining circuit to the detector, modulation corresponding to that introduced at the same station.

10. The method of secret signalling between two stations each equipped with a detector and with apparatus, including a frequency determining circuit, forming with the intervening space therebetween an end-to-end oscillator which comprises modulating the system at one station by varying the frequency of the frequency determining circuit at that station in accordance with audio frequency signal energy introduced at that station, simultaneously rendering the detector at that station unresponsive to modulation of the system resulting from such Variation of the frequency determining circuit, similarly and concurrently modulating the system at the other station by varying the frequency determining circuit at the other station in accordance with audio frequency energy introduced vat the other station and rendering the detector at the other station unresponsive tothe modulation of the system resulting from vsuch variation of the frequency determining circuit at the 'other station.

11. The method of secret signalling between two stations each equipped with a source of audio frequency signal energy, a pair of reactance tubes and a detector connected with one of said tubes and equipped with apparatus, including a frequency determining circuit, forming with the intervening space therebetween an end-to-end oscillator which comprises energizing one reactance tube at each station with energy from the source of audio frequency signal energy to cause variation in the tuning of the frequency determining circuit in the oscillatory chain at that station, simultaneously energizing the other reactance tube at the same station with energy from the same source and adjusting the proportion of the audio frequency energy delivered to the other reactance tube until the detector is unresponsive to frequency modulation of the system resulting from the energization o'f the first reactance tube.

12. The method of secret signalling between two stations each equipped with apparatus including a pair of reactance tubes, an oscillator and a detector and with other apparatus including 'a frequency determining circuit shunted -by one of the reactance tubes, such other apparatus forming with the intervening space between the vstations an end-to-end oscillator, which comprises utilizing audio frequency signal energy at each station to frequency modulate the system-simultaneously by varying the tuning of the frequency determining circuit at each station through the reactance tube shunting the circuit, controlling the frequency of the oscillator by energizing the other reactance tube at each station with audio frequency signal energy, mixing energy 4from the frequency determining circuit with energy delivered by the oscillator, delivering the mixed energy to the detector and nally adjusting the relative magnitude .of the audio frequency energy applied to the 4two reactance vtubes at one station until there is no response in the detector at that station when no modulation is :introduced into the system at the other station.

13. The method of communicating from one station to another station equipped with apparatus, including frequency determining circuits, forming together with the intervening space an end-to-cnd oscillator, which comprises frequency modulating the end-to-end oscillator by varying the frequency determining circuit at the one station in response to audio frequency signal energy, detecting at the latter station the frequency modulation introduced at the former station, reproducing the intelligence so detected at the latter station and `retransmitting the frequency modulated energy.

14. A two-way communication system having at least two stations, each vstation comprising an interconnected receiver and transmitter, a source of signal energy, a detector for demodulating energy received from the companion station, and means including a mixer connected between the detector and the receiver and transmitter to render the detector unresponsive to said source of signal energy.

215. A Itwo-Way communication system having at least two stations, each station comprising an interconnected receiver and transmitter, a, source of signal energy, means for frequency `modulating the transmitter with the signal energy, a frequency deviation detector for demodulating the signal from the companion station, and means including a mixer between the detector and the receiving and transmitting equipment to render the detector unresponsive to said source of signal energy.

16. In a communication system having at least two stations whereby intelligence may be simultaneously transmitted from each station to the other, the method of signalling comprising modulating a carrier with intelligence at one station and transmitting it to the other station, receiving the carrier at said other station and demodulating a part thereof to reproduce the intelligence transmitted from said one station, amplifying the other part of the carrier and modulating it with intelligence at said other station for transmission to said one station, preventing the reproduction at said other station of its signal intelligence by mixing a similar but out-of -phase signal, receiving the carrier at said one station and treating it in the manner set forth for the other station for reproducing the received intelligence and transmitting intelligence to the other station.

17. In a communication system having at least two stations whereby intelligence may be simultaneously transmitted from each station to the other, .the method of signaling comprising frequency modulating a carrier with intelligence originating at one station and transmitting it to the other station, receiving the carrier at said other station, frequency modulating the carrier with intelligence at said other station and amplifying it for transmission to said onestation, preventing the reproduction at said one station of its signal intelligence by mixing -a similar but out-of-phase signal therewith, receiving the carrier at said other station and treating itin the manner set forth for said one station for reproducing the received intelligence and transmitting intelligence to the other station.

18. In a secret signaling system for the simultaneous transmission of intelligence from each station to the other, the method of treating signals at each station comprising receiving a carrier frequency at one station from the other station frequency modulated by audio frequency intelligence, frequency modulating the received signal with a source of audio frequency intelligence at said one station, generating a signal at said one station having a frequency equal vto the carrier frequency of the received signal and frequency modulating it with its audio frequency intelligence, mixing said received and modulated carrier in an out-of-phase relationship with said .modulated generated signal, demodulating said mixed signals to reproduce the intelligence impressed on the received carrier at said other station, and

amplifying the received and modulated carrier and transmitting it to said other station.

19. In a station-to-station secret signaling system of the type wherein equipment at the two stations together with the intervening space form an end-to-end oscillator, the combination which comprises a frequency determining circuit at each station in the oscillatory chain, means at each station for frequency modulating said circuit in response to audio frequency signal energy, means at each station connected to said circuit for reproducing received signal energy from the other station including signal energy suppressing means under the control of a version of the audio frequency signal energy originating at the receiving station for cancelling changes in frequency of energy passed by said circuit and produced by the modulating means at the same station.

CARL EDWARD ATKINS. ALEXANDER V. MITCHELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

